Motor control system



Nov. 3, 1942. J. a. STEPHENSON MOTOR CONTROL SYSTEM Filed July 12, 1941 NWUKUN lNVENTOR 0 mm. 6 L

ATTORNEY- Patented Nov. 3,, 19 42 I MOTOR CONTROL SYSTEM James G. Stephenson, Wiikinsburg, Pa., assignor to Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsylvania Application July 12, 1941, Serial No. 402,124

7 Claims.

My invention relates to motor control systems and more particularly to a system of control for varying the speeds of an electric motor.

In the paper industry and similar industries, webs of paper or other materials are often required to be wound into large rolls. The paper is supplied to the winder roll at a constant speed but as the roll diameter increases the motor speed driving the roll has to be decreased to match the changes in roll diameter.

One object of my invention is the provision of means for maintaining the tension in a web of material being wound on a roll constant.

Another object of my invention is the provision of decreasing the normal speed of a motor in response to the rising changes of an electric quantity without a consequent appreciable increase in motor speed in response to a decreasing change in the electrical quantity.

The objects hereinbefore expressed are merely illustrative of numerous other objects and advantages that will, no doubt, become readily apparent from a study of the following specification and the accompanying drawing, in which:

The single figure is a diagrammatic showing of one embodiment of my invention.

In the single figure showing my invention diagrammatically, the reference character M designates a motor coupled to drive a roll R on which a sheet of web material P, as paper, cloth, rubber, etc., is being wound. The web, or sheet of paper, is supplied to the roll at a constant speed and as the diameter of the roll of paper on roll R increases, it is necessary that the speed of the motor M be automatically decreased in direct proportion to the increase in roll diameter to prevent undue stresses in the paper.

The prior art devices for decreasing the motor speed usually include the use in each case of a regulator that operates to maintain constant current in the motor armature circuit. As the roll diameter increases and the current in the armature circuit tends to increase, the regulator op-.

erates to increase the motor field fiux, thereby causing the motor to run at a lower speed with the same armature current.

The prior art devices have, however, the bad may, and usually is, a dangerous speed, depending on the of the roll at the time the paper breaks and on the operating voltage across the motor armature.

In the single figure, 1' illustrate a simplified form of a regulating scheme that makes use of a rotating current regulator and control that do not produce the dangerous operating characteristics of the motor produced by the devices of the prior art. When the paper breaks, there is no appreciable increase in motor speed.

In my scheme of control, the motor M has two fields l3 and 31. Field i3 is excited from the constant potential buses 2 and 5 through the motor operated rheostat I2. The field 31 is excited from the rotating regulator generator or exciter E. The operation of generatorE on this field 31 is such that substantially constant ourrent is maintained in the armature of motor M.

The generator E has three fields as follows: The field 38' is a self-exciting field and .is in the series circuit including the armature of E, contact members 2 J, rectifier 38, field 31, field 38, and the resistor 39. The field 3 is excited from constant potential buses 2 and 5 and in effect is opposite to the effect of field 38 as indicated by the arrows adjacent these fields. connected across the resistor 2! connected in the loop circuit of generator G and motor M, and its effect is ameasure of the current to be held constant. A satisfactory condition for stable operation and effective regulation is such that the ampere turns in field 3 must be equal and opposite to the ampere turns in field 34.

On starting a roll of paper, when the diameter of the roll R is small, the speed of motor M must be high and is adjusted by proper position-' ing of lead 5i on resistor l2 so that field i3 produces the desired high speed. On. the other hand a proper adjustment of lead 44 on resistor 48 determines the rapidity of response of the motor 50 thereby giving the desired quick control of the resistance in the field circuit of the field l3. This will become more apparent as the description proceeds. As paper is wound on the roll and its diameter increases, there is a tendency for the current to increase in the armature circuit of motor M. This causes an increased voltage drop across resistor 21 with a consequent rise in the excitation of field 34. The regulator generator voltage thus rises with a consequent increase' in the field excitation of the field 31 of motor M. This increased current in the motor field 31 causes the motor M to be operated at a slower speed with the same armature current.

The additional current through the field 31 Field 34 is also fiows through the regulator generator field 38 causing the generator to maintain itself at the higher voltage.

The speed regulating control relay 42 is a calibrated relay set to pick up at some selected voltage across the regulator armature and to drop out at a definite but slightly lower voltage of the regulator generator. When the regulator generator voltage builds up to the pick-up voltage, relay 42 operates to close contact members 43. A circuit is thus established from bus 2 through conductor 45, contact members 43, field 46 and armature 4I, resistor 48 and speed adlusting lead 44 to bus 5.

The rheostat motor 50 thus operates rheostat arm I I clockwise to shunt resistor sections of the speed adjusting rheostat I2. The excitation of field I3 is thus increased and the motor M slows down still more. As the motor slows down because of the increased excitation in field I3, the motor armature current decreases and the motor speed tends to decrease. As the motor armature current decreases, the excitation of field 34 decreases, the voltage of regulator generator E decreases and in consequence the exciting current in motor field 31 decreases and the exciting current in field 38 decreases, with the result that the motor armature current is maintained constant and thus the tension in the paper, P, is

maintained constant.

This effectv of decreasing the voltage of thegenerator E continues'until the dropout voltage are thus opened and motor 50 stops. As the roll diameter increases further, the cycle of operation repeats itself. At each operation of relay 42, the rheostat arm II is moved to a new position oi adjustment. This sequence continues until a full roll of paper is reached, when the motor M is running at its slowest speed.

With all prior art devices where the regulator attempts to control the motor speed over its entire range when the paper breaks the motor, as M, immediately speeds up to its maximum value. This resultant high speed is that which would be necessary for a small roll, namely the speed determined by the adjustment of lead i on the field resistor I2.

With my system of speed regulation, the speed increases, upon a break of the paper, is limited in all cases to that which results when the excitation of field 31 is removed. This value can be changed at will by calibrating relay 42 to'pick up at a higher or lower voltage, as desired.

The action of the rotating regulator generator and the devices it controls when the paper breaks is to remove all excitation from field 31 and even to reverse its polarity. To prevent reversal of polarity, I use the electric valve or rectifier 38. This rectifier permits flow of current in the motor field 31 and to the coil 4| of relay 42 in one direction only. Thus it will be seen that the greatest speed increase that can result at any time upon a break of the paper, is that which is caused by the removal from field 31 of the excitation being supplied by the regulator at the particular instant when the paper breaks. In other words, the maximum speed of the motor M at the moment of the breaking of the paper is determined by the position of rheostat arm II at the time and not as with prior art devices upon the maximum speed adjustment determined by the lead 5 I.

With my system of control, I obtain an increase or decrease in regulator voltage by shifting the lead 52 on the tension rheostat 4, which thus effects a readjustment on the regulator field 3. An increase in ampere turns in this field 3 means that higher ampere turns in field 34 are necessary in order to produce a balanced condition. This increase represents an increase in motor armature current.

When a full roll has been reached and a new one is to be shorted, the Increase' push button is depressed to cause the motor 5|) to move arm I I counterclockwise to'the weak field position or move opposite the connection of lead 5| to insert the maximum number of resistor sections in the field I3. If for-any reason, the arm II is to be moved clockwise independent of automatic control, the Decrease push button is depressed.

A still better understanding of the novel features of my invention may be obtained from a study of typical operations. The motor A is a large constant speed motor operating, when circuit breaker B is closed, a number of winders, but in the figure is shown coupled to drive only the regulating exciter, E, and the generator, G.

Initially the motor M is operated slowly by suitable inching means (not shown) to thread the paper on the roll. To obtain the desired operations lead 44 is adjusted to get a rheostat motor speed having, preferably, a speed somewhere nearly proportional to the paper speed. Lead BI is adjusted to get the maximum allowable speed of roll R when the roll is substantially empty, that is, when the winding is to start after the paper is threaded on the roll. This speed will, of course, be matched to the paper speed. Lead 52 is adjusted to get the proper paper tension.

After the necessary adjustments, which usually need not be altered for the winding of successive rolls, except where entirely different materials are to be wound on roll R, the attendant closes switch .I to energize the control circuits, and operates switch 26 to connect my system for automatic speed control. Operation of switch I establishes one circuit from energized bus 2 through field 3, tension control lead 52 of the rheostat 4 to bus. Field I and the series connected resistor '5 are connected in parallel to all the resistor sections of rheostat 4 and are thus also energized.

Another circuit is established from bus 2 through adjustable resistor 8, neutralizing coil 8 of the time limit relay ID to bus 5. A further circuit is established from bus 2 through the rheostat arm I I, all of the selected usable resistor sections of rheostat I2, and fie d I3 to bus 5.

To start the operation starting swit h I 5 is actuated whereupon a circuit is established from bus 2 through stop switch I4, starting switch I5, magnetizing coil I6 of relay I II, back contact members I! of the main contactor I8, to bus 5.

The time limit relay operates immediately to close contact members I9 to energize the actuating coil 20 of the main contactor and at the same time open the contact members 33. Operation 01' main contactor I 8, closes contact members 23 to establish its own holding circuit, closes contact members 29 to connect the motor M to the generator G, closes contact members 2| to complete the circuit through coil 24, of control relay 25, and automatic control switch 26, and opens contact members I! to deenergize the magnetizing coil of relay I0.

Energization of coil 24 causes the immediate closing of contact members 35, whereas the closing of contact members 2-9 establishes a circuit tact members 29, stabilizing series field 30, mo:

tor M to the negative terminal of the generator G.

After a predetermined time interval, depend-- ing on the adjustment of spring means 3|, the delaying effect of the short-circuiting winding 37., and the neutralizing coil 9, contact members 33 close thus bringing the motor up to full speed.

The regulating control will :be apparent from the explanations already given.

' The system of control I have disclosed is merely illustrative of my invention and I, therefore, do not wish to be limited to the particular showing made but wish to be limited only by the scope of the claims hereto appended.

I claim as my invention:

1. In a system of control for a direct current motor, in combination, a source of constant direct current potential, a motor having anarmature winding connected to the said source of constant potential and having two field windings, means for energizing the field windings, means responsive to the current variations in the armature winding of the motor for varying the excitation of one of the field windings in correspondence to the variations in armature current, means responsive to a predetermined rise in excitation in the excitation in the said one field windin for increasing the excitation in the other of said field windings and means responsive to a predetermined decrease in the excitation in the said one field winding for stopping the rise in excitation in the other of said field windings.

2. In a system of control, in combination, a loop circuit, including a generator armature winding and a motor armature winding, for carry ing currents subject to variations in value due to changes in the load driven by the motor inthe motor, means responsive to variations in the value of the current in the loop circuit for correspondinglyyarying the current in the said field winding, a second field winding for the motor, and means'responsive only to a predetermined rise in the current value in the loop circuit for correspondingly only increasing the current in the second field winding.

3. In a system of control, in combination, a loop circuit, including a generator armature winding and a motor armature winding, for carrying currents subject to variations in value due to changes in the load driven by the motor included in the loop circuit, a field. winding for the motor, means responsive to variations in the value of the current in the loop circuit for correspondingly varying the current in the said field winding, a second field winding for the motor, means responsive only to a predetermined rise in the current value in the loop circuit for correspondingly only increasing the current in the second field winding, and means responsive to a predetermined decrease of the current in he first field winding for stopping the means .50: increasing the current in the second field winding.

cluded in the loop circuit, a field winding for 4. In a system of control for a direct current electric motor, in combination, a generator coupled to be driven at a substantially constant speed, a motor having an armature connected in a loop circuit with the generator and having two field windings, means responsive to varia tions in current in the loop circuit for changing the excitation of one of the field windings, means responsive only to a predetermined rise in current in the loop circuit for only increasing the excitation of the other field winding, and means responsive to a predetermined decrease in the current fiowing in the first field winding for stopping the means for increasing the current in the second field winding.

5. In a system of control for a direct current electric motor, in combination, a generator coupled to be driven at a substantially constant speed, a motor having an armature connected in aloop circuit with thegenerator and having two field windings, means responsive to varia tions in current in the loop circuit for changing the excitation of one of the field windings, and means responsive only to a predetermined rise in current in the loop circuit for only increasing mined rise in current in said circuit for increasing the current in the third electromagnetic winding, and means responsive to a predetermined decrease in the current in the second named electromagnetic winding for stopping the operation of the means for increasing the cur rent in the third electromagnetic winding.

7. In a system of control, in combination, a motor subject to variations in load current, an exciter driven at a constant speed having three field windings, one of the field windings being energized a constant amount, said motor having two field windings one of the motor field windings being connected in series with the ex- 50- citer and one of the other orsecond field windings of the exciter, the third field winding,'normally equal and opposite in effect to the effect of the first exciter field windings and cumulative to the second exciter field winding, being connected to the motor load circuit to thus vary with variations in the motor load, whereby the exciter voltage is changed proportional to the change in excitation in the excitation of one of the motor field windings, means responsive to a predetermined use inthe excitation in the said one moe tor field winding for increasing the excitation of.

the second motor field winding, and responsive to a predetermined drop in excitation of the first motor field winding for stopping the rise in excitation in the second motor field winding.

' JAMES G. STEPHENSON. 

